Sensorless brushless motors include a stator that most commonly includes three phase windings in a star configuration and a permanent magnet rotor. Through appropriate excitation sequences of the stator windings, a rotating magnetic field that causes attraction-repulsion forces on the permanent magnet of the rotor is produced. Such an electric machine is synchronous and to work correctly the stator rotating magnetic field should maintain a certain phase lag from the rotor magnetic field. For ensuring this condition it may be necessary to know at any instant the rotor angular position and to consequently drive the stator windings.
In sensorless motors, the back electromotive force (BEMF) induced on the stator windings is detected for estimating the rotor position. However, the amplitude of the BEMF is proportional to the speed of the rotor (BEMF=Ke*speed), thus it can hardly be detected if the rotor speed is too low, as at start up. To overcome this issue, it is necessary to rotate the rotor at an angular speed sufficient to generate a BEMF that may be reliably detected. Methods that address this need are commonly referred as “startup” procedures. Once a certain speed threshold is reached with the startup procedure, the excitation sequence of the stator windings is synchronized with the rotor position reconstructed from the BEMF detection and the sequential excitation of the stator windings is performed in a “BEMF-closed-loop” mode.
Numerous startup techniques are known; some carried out completely in open loop mode, other exploiting a feedback signal (closed-loop startup). In closed loop startup procedures, the feedback signal represents the position of the rotor reconstructed from indirect (sensorless) measurements and processing.
A common method of determining the position of the rotor at very low speed (or at standstill) is based on analyzing the current in the windings following the application of a voltage step (known as “inductive sensing”). The functioning principle rests on saturating the magnetic circuit causing a consequent modification of the inductance of the phase windings of the motor. From an analysis of the current response to a voltage step it is possible to determine unambiguously the position of the rotor. Once the position of the rotor is acquired, the stator windings are sequentially driven to accelerate the rotor and bring it to a speed sufficiently large to render detectable back electromotive forces (BEMF). Once the rotor reaches a speed sufficient to make the BEMF reliably detectable, the position of the rotor begins to be constantly detected and the driving of the phase windings may continue in BEMF-closed-loop mode.
These start up methods based on the analysis of currents require a direct or indirect measurement of the current flowing through the phase windings and a sufficiently accurate current sensing circuit. Typically, a sense resistor is used, but this may result in an added cost. Other methods use current sensors integrated within the power devices (sense-FET) but sensing precision (matching among distinct sense-FETs) is often insufficient.
Another method, based on saturating the magnetic circuit, that does not require direct current measurements was disclosed by the present Assignee in U.S. Pat. No. 6,841,903. According to this method, the position of the rotor is unambiguously determined by measuring the re-circulation time of the phase currents. A general drawback of methods based on saturating the magnetic circuit is the relatively long time required for the measurements. Stator currents must saturate the magnetic circuit, thus it may be necessary to force relatively large currents that in turn cause relatively long excitation times and thus expand latency times typically in the order of several hundreds microseconds, for determining the rotor position. This latency causes a phase error (when the rotor is already moving) and limits the maximum speed attainable at the end of the startup procedure making more critical the determination of the speed at which handover from the startup mode to the BEMF-closed-loop control mode should take place.
Another drawback of prior art methods based on the saturation of the magnetic circuit is sensitivity to mismatches among the motor windings, because estimation of the rotor position takes place by comparing measurements relative to different windings and results of the comparisons also depend on the match among motor windings.
Yet another known approach is based on measuring the mutual inductance through a measurement of the induced voltage by transformer effect, obtained by processing the signal sensed on the non-conductive or non-excited phase winding of the motor (the winding that is momentarily switched to a high impedance condition) when current flows through the other two phase windings of the three phase motor. According to the method described in U.S. Pat. No. 6,555,977, estimations of the rotor position are made by canceling undesired components of the induced voltage superposed to the mutual inductance component, through an appropriate filtering. This method may require a dedicated filtering stage that may increase circuit complexity and costs.